The present invention relates to a rectifying circuit for converting an N-phase (N being a natural number equal to or greater than 2) AC voltage into a DC voltage, as well as a control method therefor.
FIG. 14 shows a conventional rectifying circuit for converting a three-phase AC voltage into a DC voltage. This conventional technique is substantially described in Japanese Patent Publication (KOKAI) No. 9-182441. In FIG. 14, R, S and T denote AC input terminals; P and N denote DC output terminals; L1, L2 and L3 denote reactors; D1 to D18 denote diodes; S1 to S3 denote switching devices of corresponding phases; and C1 and C2 denote capacitors.
Next, the operation of this circuit will be described. In the following, names of elements may be omitted, with only their reference numerals shown. For example, when the switching devices S1 and S2 are turned on, current flows through a path Rxe2x86x92L1xe2x86x92D1xe2x86x92S1xe2x86x92D8xe2x86x92D9xe2x86x92S2xe2x86x92D4xe2x86x92L2xe2x86x92Sxe2x86x92R to accumulate energy in the reactors L1 and L2. Furthermore, when S1 is turned off with S2 on, the energy from the reactors L1 and L2 is charged to the capacitor C1 through the path Rxe2x86x92L1xe2x86x92D1xe2x86x92D13xe2x86x92C1xe2x86x92D9xe2x86x92S2xe2x86x92D4xe2x86x92L2xe2x86x92Sxe2x86x92R. On the other hand, when S2 is turned off with S1 on, current flows through a path Rxe2x86x92L1xe2x86x92D1xe2x86x92S1xe2x86x92D8xe2x86x92C2xe2x86x92D16xe2x86x92D4xe2x86x92L2xe2x86x92Sxe2x86x92R to charge the energy from the reactors L1 and L2 to the capacitor C2.
Further, when both S1 and S2 are turned off, current flows through a path Rxe2x86x92L1xe2x86x92D1xe2x86x92D13xe2x86x92C1xe2x86x92C2xe2x86x92D16xe2x86x92D4xe2x86x92L2xe2x86x92Sxe2x86x92R to charge the energy from the reactors L1 and L2 to both capacitors C1 and C2. Repetition of such switching operations enables an AC voltage to be converted into a DC voltage while the input current is controlled at a high power factor. Further, the voltages of the two capacitors C1 and C2 can be individually regulated by adjusting the on-time of the switching devices S1 to S3.
In the conventional three-phase input technique shown in FIG. 14, the number of semiconductor devices (switching devices and diodes) through which current passes is six for storage of energy in the reactors, five for individual charging of the capacitor C1 or C2, and four for the simultaneous charging of both capacitors C1 and C2. Thus, current passes through a large number of devices, thus increasing energy loss in the semiconductor devices. Further, the cooling parts for reducing heat generated by the energy loss must be large, thereby increasing the size and price of the apparatus. Additionally, a main circuit includes a large number of semiconductor devices, specifically 21.
It is thus an object of the present invention to provide a rectifying circuit and control method therefor for reducing the number of semiconductor devices through which current passes as compared to the prior art, thereby reducing energy loss, and for reducing number of semiconductor device parts, making it possible to reduce the size, weight, and price of the apparatus.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To attain the above object, the first aspect of the present invention is a rectifying circuit for converting an N-phase (N being a natural number equal to or greater than 2) AC voltage into a DC voltage. In the rectifying circuit, a series circuit of two switching devices having the same conductive direction and a series circuit of two diodes having the same conductive direction are connected together in parallel to constitute a bi-directional switch circuit for one phase. N bi-directional switch circuits are provided, and a junction between the switching devices of each bi-directional circuit is connected to an AC input terminal of a corresponding phase via a reactor. A cathode of the diode series circuit of each bi-directional switch circuit is connected to a positive DC output terminal via a diode, and an anode of the diode series circuits of each bi-directional switch circuit is connected to a negative DC output terminal via a diode. Two capacitors are connected in series between the positive and negative DC output terminals. The junction between the diodes of each bi-directional switch circuit is connected to a junction between the two capacitors.
In the second aspect of the present invention, two series circuits each comprising a diode and a switching device having different conductive directions are formed, and the first and second series circuits are connected together in parallel so that the cathodes of the diodes are connected together to constitute a bi-directional switch circuit for one phase. N bi-directional switch circuits are provided, and an internal junction of the first series circuit of each of the bi-directional switch circuits is connected to an AC input terminal of a corresponding phase via a reactor. The cathodes of the diodes of each bi-directional switch circuit are connected to a positive DC output terminal via a diode, and a junction between the switching devices of each bi-directional switch circuit is connected to a negative DC output terminal via a diode. Two capacitors are connected together in series between the positive and negative DC output terminals. All internal junctions of the second series circuits of the bi-directional switch circuits are connected to a junction between the two capacitors.
In the third aspect of the present invention, two series circuits each comprising a diode and a switching device having different conductive directions are formed, and the first and second series circuits are connected together in parallel so that the anodes of the diodes are connected together to constitute a bi-directional switch circuit for one phase. N bi-directional switch circuits are provided, and an internal junction of the first series circuit of each of the bi-directional switch circuits is connected to an AC input terminal of a corresponding phase via a reactor. A junction between the switching devices of each bi-directional switch circuit is connected to a positive DC output terminal via a diode, and the anodes of the diodes of each bi-directional switch circuit are connected to a negative DC output terminal via a diode. Two capacitors are connected together in series between the positive and negative DC output terminals via a diode. All internal junctions of the second series circuits of the bi-directional switch circuits are connected to a junction between the two capacitors.
In the fourth aspect of the present invention, a series circuit of two diodes having the same conductive direction and a series circuit of two switching devices having the same conductive direction are connected together in parallel to constitute a bi-directional switch circuit for one phase. N bi-directional switch circuits are provided, and a junction between the diodes of each bi-directional switch circuit is connected to an AC input terminal of corresponding phase via a reactor. A cathode of the diode series circuit of each bi-directional switch circuit is connected to a positive DC output terminal via a diode, and an anode of the diode series circuits of each bi-directional switch circuit is connected to a negative DC output terminal via a diode. Two capacitors are connected in series between the positive and negative DC output terminals, and a junction between the switching devices of each bi-directional switch circuit is connected to a junction between the two capacitors.
In the fifth aspect of the present invention, according to any of the second to fourth aspects, one of the two diodes constituting each bi-directional switch circuit is replaced with a thyrister and, when the interphase power is low, the thyrister is turned on to restrain a rush current to the capacitor when the power supply is turned on.
In the sixth aspect of the present invention, N+1 series circuits each comprising two switching devices are formed. The first to N+1-th switching-device series circuits are connected together in parallel, and a diode is connected inversely in parallel to each switching device. N series circuits each comprising two diodes are formed. The first to N-th diode series circuits are connected together in parallel so that the cathodes of the diodes are connected together, while the anodes thereof are connected together and the cathodes are connected to a positive DC output terminal. The anodes are connected to a negative DC output terminal. Two capacitors are connected in series between the positive and negative DC output terminals. A junction between the diodes of each of the first to N-th diode series circuits is connected to a junction between the switching devices of a corresponding one of the first to N-th switching-device series circuits, and the junction between the diodes of each of the first to N-th diode series circuit is connected to an AC input terminal of a corresponding phase via a reactor. A junction between the switching devices of the N+1-th switching-device series circuit is connected to a junction between the two capacitors.
In the seventh aspect of the present invention, N+1 series circuits each comprising two switching devices are formed. The first to N+1-th switching-device series circuits are connected together in parallel, and a diode is connected inversely in parallel to each switching device. The cathodes of the diodes of parallel-connection circuits of the first to N+1-th switching-device series circuits are connected to a positive DC output terminal via a diode, and the anodes of the diodes of the parallel-connection circuits of the first to N+1-th switching-device series circuits are connected to a negative DC output terminal via a diode. Two capacitors are connected in series between the positive and negative DC output terminals. A junction between the switching devices of each of the first to N-th switching-device series circuits is connected to an AC input terminal of a corresponding phase via a reactor. The junction between the switching devices of the N+1-th switching-device series circuit is connected to a junction between the two capacitors.
In the eighth aspect of the present invention, in the rectifying circuit according to any of the first to seventh aspects, detected voltage values for both capacitors connected between DC output terminals are fed back in response to each command value so that, on the basis of the deviation between the detected voltage values and the command value, the switching devices present in the corresponding charging paths of the two capacitors are independently controlled to individually control the voltages at the two capacitors.